Relative demensions of J tube

I've been experimenting with building rocket stoves using a combination of concrete blocks and a clay based mortar. The concrete blocks are the type that have two square holes that run from side to side or top to bottom, cinder blocks I believe they are called. Anyway my first attempt was very successful but my latest modifications have proved less that effective... as is sometimes the way. I feel I know what the issue is and can easily make a few changes that should sort things out.

What I was wondering is can anyone please tell me the relative (optimum) dimensions of the 'J' tube? Can someone confirm that I have this right as in all the literature that I have, these dimensions seem a little vague and at times just plain confusing!

Area of cross section of 'Feed Tube' = Area of cross section of 'Burn Chamber' = Area of cross section of 'Heat Riser' - is this correct? I did see somewhere that these are relatively flexible so long as the diameter of the 'Burn Chamber' is ALWAYS the smallest cross section. In my example, opening of 'Burn Chamber' = 7x7 (inches) = Area of cross section of 'Burn Chamber' = 49 (sq inches)

Height of 'Heat Riser' (measured from the top edge of the 'Burn Chamber' to the exit) I believe this is relative to the diameter of the 'Burn Chamber' = Minimum height being half of the 'Burn Chamber' cross section in my case 25 inches and a maximum height equal to that of the area of the cross section of the 'Burn Chamber' in my case 50 inches. I understand that having the 'Heat Riser' on the shorter side makes things more smoky but hotter for cooking etc and having it on the high side burns more efficiently (less smoke) but is less effective for heating. I was looking at about 30 - 35 inches in my next modification depending on how the blocks go together. This is the relative dimension that I really don't seem to be able to nail down in any of literature.

Length of 'Burn Chamber' (measured from opening to 'Feed Tube' to the edge of the up turn) = half the height of 'Heat Riser'. Again in my case the 'Heat Riser' is about 30 inches so my 'Burn Chamber' should be about 15 inches. This I am pretty sure is correct.

Depth of 'Feed Tube' = as short as possible. The top of my 'Feed Tube' will be about 2 inches above the top of the entrance to the burn chamber. Does the 'Feed Tube' have any bearing on the draw of the system? I'm guessing so but I don't seem to be able to find out what changing the height of this would to the efficiency of the stove.

Having these relative dimensions and the effects that changing them have on the function of the stove would be really useful and I would be really grateful if someone can confirm what I have said or put me on the right tracks.

Thanks Folks
Burt

Burt,
I am also very new to these and have many of the same questions as you. The dimensions and how they are used is my main area of confusion. I would also love to hear from some of the experienced people to get some real examples of what works and what does not. When I do get to the point of build my first Heater it will be with fire brick so I would like to get it right the first time.

Hopefully someone can help us with the proper dimensions.

Hi Burt; I have only built one rmh so I am far from an expert at this but i'll try to help. First off I should ask if you have a copy of ianto evans book rocket mass heaters ? If you don't you should have one, it is available on line as a book or as a pdf copy. My J tube is a cast core rather than firebrick , but the dimensions should remain the same. My riser and feed tube are 5 1/2 " taller than the top of the burn tunnel , the burn tunnel is 9 1/2 " long from inner edge to inner edge or a total of 24" long end to end. The riser itself is apx 42" tall (also cast using fireclay & perlite in a cardboard concrete form ), on the feed tube side I used a cut down 5 gal metal can to raise up about a foot , this can was then cobbed over. I am unhappy with the metal can in this spot and I will replace it this summer with an 8" square clay flue liner about 12" tall. The expansion rate of metal is different than clay causing the can to come loose from the cob. With my 8" system I am getting temps at the barrel top of 1100 degrees (glowing orange) and side of barrel is 950, they work really well !!! Oh Should add that your cinder blocks are not going to hold up to the heat. Good luck Tom

Thomas, thank you very much for the information and measurements, I've downloaded a copy of Ianto Evans' book and will read through it. I'll try and replicate your dimensions or close to them in my next experiment. The last stove that I built was by far the most efficient and produced far more heat all round, that said the dimensions where a little large than previous attempts and thus the fire too was a little larger, I don't know if this would account for the greater heat or not, I did use a longer Heat Riser (I found a 40" metal tube that I'm using) and the additional draw really got the rocket going.

Klorinth, I have now tried several different designs and proportions all using the cinder blocks that I have found around the farm, I have posted photos and commentaries on their construction and efficiency on Facebook (Rocket Stoves) please feel free to take a look. I have also tried to incorporate various uses for Stoves as well. If I get the time I'll try and post all the pictures and commentaries on this thread.

I'll keep you updated with how things go

I recently downloaded the book. If you are wanting to understand the important dimentions skim through the book and the info you want is on two pages, find the pages and spend 20minutes understanding those two pages and you will be glad you bought it. And things will be clearer.

I found said pages last night Lenny. It's an excellent book and looks like a real gem when it comes to Rocket Mass Heaters.

It would still be interesting to know what the effects of changing the relative ratios are, especially as I'm also experimenting with making multifunctional use Rocket Stoves.

When you are thinking of your multi-functional rocket heaters, separate the combustion function from whatever you want to do with the heat. Make this separation clearly in your mind first and then in your designs.

This is an important principle. If you ignore it and try to take heat before the combustion is done, your fire will be cooler and consequently, less efficient. You will chop more wood for the same heat and create more air pollution.

If you want to innovate with rocket heaters, you would do well to start with the dimensions and ratios which have already been optimized by lots of other people.

Okay Cindy that makes sense to me, in one experiment I did 'play' with directing the heat back down and around the Burn Chamber in the thought that the extra heat around the Burn Chamber would improve efficiency. However this was early on before I realised the importance of the relative dimensions. That particular design had very poor dimensions even so the extra heat did in fact help and did compensate to a degree the fact that the dimensions where all to cock. It's been a good learning curve for me and I am now realising what works and what doesn't, where I have a little leeway and where I don't, and keeping the Burn Chamber as hot as possible is one of the areas where there is little room for manoeuvre.

Thanks for the pointer.
Burt